Add PMIx plugin

Proof of concept for now.

cmake/FindPMIx.cmake

@@ -0,0 +1,67 @@
+################################################################################
+#    Copyright (C) 2019 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH    #
+#                                                                              #
+#              This software is distributed under the terms of the             #
+#              GNU Lesser General Public Licence (LGPL) version 3,             #
+#                  copied verbatim in the file "LICENSE"                       #
+################################################################################
+
+find_path(PMIx_INCLUDE_DIR
+  NAMES pmix.h
+  HINTS ${PMIX_ROOT} $ENV{PMIX_ROOT}
+  PATH_SUFFIXES include
+)
+
+find_path(PMIx_LIBRARY_DIR
+  NAMES libpmix.dylib libpmix.so
+  HINTS ${PMIX_ROOT} $ENV{PMIX_ROOT}
+  PATH_SUFFIXES lib lib64
+)
+
+find_library(PMIx_LIBRARY_SHARED
+  NAMES libpmix.dylib libpmix.so
+  HINTS ${PMIX_ROOT} $ENV{PMIX_ROOT}
+  PATH_SUFFIXES lib lib64
+)
+
+find_file(PMIx_VERSION_FILE
+  NAMES pmix_version.h
+  HINTS ${PMIX_ROOT} $ENV{PMIX_ROOT}
+  PATH_SUFFIXES include
+)
+
+file(READ "${PMIx_VERSION_FILE}" __version_raw)
+string(REGEX MATCH "#define PMIX_VERSION_MAJOR ([0-9]?)L?"
+  __version_major_raw "${__version_raw}"
+)
+set(PMIx_VERSION_MAJOR "${CMAKE_MATCH_1}")
+
+string(REGEX MATCH "#define PMIX_VERSION_MINOR ([0-9]?)L?"
+  __version_minor_raw "${__version_raw}"
+)
+set(PMIx_VERSION_MINOR "${CMAKE_MATCH_1}")
+
+string(REGEX MATCH "#define PMIX_VERSION_RELEASE ([0-9]?)L?"
+  __version_patch_raw "${__version_raw}"
+)
+set(PMIx_VERSION_PATCH "${CMAKE_MATCH_1}")
+
+set(PMIx_VERSION "${PMIx_VERSION_MAJOR}.${PMIx_VERSION_MINOR}.${PMIx_VERSION_PATCH}")
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(PMIx
+  REQUIRED_VARS
+  PMIx_INCLUDE_DIR
+  PMIx_LIBRARY_DIR
+  PMIx_LIBRARY_SHARED
+
+  VERSION_VAR PMIx_VERSION
+)
+
+if(NOT TARGET PMIx::libpmix AND PMIx_FOUND)
+  add_library(PMIx::libpmix SHARED IMPORTED)
+  set_target_properties(PMIx::libpmix PROPERTIES
+    IMPORTED_LOCATION ${PMIx_LIBRARY_SHARED}
+    INTERFACE_INCLUDE_DIRECTORIES ${PMIx_INCLUDE_DIR}
+  )
+endif()